If you want to xor arbitrary buffers, you don't have to treat them as numbers.

my $var1=pack 'H*','000080506300835700AD00EF';
my $var2=pack 'H*','042426FFFFFFFFFFDE00BE00';
my $result = $var1 ^ $var2;
print unpack 'H*',$result;
# 0424a6af9cff7ca8deadbeef
[download]

edit: BrowserUK++ beat me to the punch! I need to stop leaving nodes open and going for a snack before writing up my reply.